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Physics Notes (January)
Basics plus Forces
-velocity is vector, speed is scalar; vectors have direction!!! Use vector diagrams to identify forces; free
body diagrams
-displacement, measured in meters, change in position
-average speed, change in displacement over time, measured m/s or km/h
-average acceleration, change in average speed over time, measured m/s²
-constant acceleration: when acceleration is constant and greater than 0; speed will increase
proportionately to time, t, with slope of the line equalling the acceleration; distance increases as t², with
slope of the curve at any time equal to velocity at said time
-N1 (law of inertia/motion): there needs to be a net force on a body/object in order for it to have
accelerate; it cannot change velocity unless there is a net force acting on it; if a body/object is moving at
a constant velocity (at rest or not) it has no net force acting upon it
-N2 (law of acceleration/force/motion): net force on object is equal to the product of the mass and
acceleration of the body (ma is fnet); empirical formula relating acceleration and force
-gravity as a force: downward force that pulls an object toward the centre of the Earth due its mass; N2:
Fy= mg where g is 9.8m/s² (acceleration due to gravity; magnitude of this force is called weight)
-Normal force (N): when standing on surface, gravity pushes down on you; you don’t go through said
surface so there must be another force pushing you up (so you stay stationary); normal meaning
perpendicular; “when a body presses against any surface (soft or rigid) the surface deforms and pushes
back with a normal force, N, perpendicular to the surface”; normal force determines amount of friction
‘felt’ by an object
-friction: asperities is the term given to surfaces appearing very smooth but under microscopic view
show a rough surface, contain many atoms each; two adjacent surfaces with asperities moving in
opposite directions relative to one another will seize to some degree when contact is made (friction!)
-static friction: when pushing or pulling a body, friction acts in the opposite direction of the applied
force; static frictional force will increase as you push/pull with greater magnitude (µ is a dimensionless
s
coefficient); in order to move said body, enough force must be applied to overcome the static friction
-kinetic friction: once body starts moving; static friction becomes kinetic friction (f k
- Galileo’s hypothesis regarding drag force and terminal velocity: all objects fall with the same velocity in
the absence of air resistance -drag force: mechanical; when a solid object moves through a fluid (thrustdrag, ^lift, weigvt);
factors affecting drag include size/shape of object, velocity/inclination to flow (motion),
mass/viscosity/compressibility (air); drag direct relation to area (double area means double the drag),
composed of surface area (As), wing area (Aw), frontal area (Af); all objects have same frontal area;
blunt object like a ball uses velocity (A (frontal area), d (drag coefficient), ρ(air density mass per volume
unit))
-forces on a falling object, with air resistance: weight is constant (mg), resistance (drag) depends on
square of velocity, motion of object (Newtons second job); when drag is equal to weight, acceleration is
zero; velocity becomes constant (terminal velocity)
-terminal velocity: motion of a falling object with air resistance (drag); when drag is equal to weight,
acceleration becomes zero; lower terminal velocity with large area or high drag coefficient; for two
objects with the same area and drag coefficient, the heavier object will fall faster (ρ density)
-skydiving: typical terminal velocity is 215 km/h, though speed divers hit around 500 km/h
-N3 (law of reciprocal action/motion): when two bodies are interacting, the forces on the bodies from
each other are always equal in magnitude and opposite in direction; results in a third law force pair
-Energy: can be put on a scale and used to predict outcome of experiments; is scalar (a NUMBER);
changes as the force on an object changes; exists in different forms (kinetic, potential, chemical,
thermal, radiation, nuclear)
-kinetic energy: motion and energy; K=0 means an object is stationary; the greater the speed of an
object the greater its kinetic energy, since K is proportional to the square of velocity; KE exists even if no
force acts on an object; units 1kg (m/s)²=1J; KE uses Joules (1GJ is 1 000 000 000J)
-work: change in kinetic energy; ‘doing work’ means transferring kinetic energy; change in kinetic energy
always associated with acceleration for an object of fixed mass...for K to change the object’s velocity
must change, and a change in velocity as in acceleration; work is associated with a force moving an
object through distance
-work energy theorem: valid for constant force with the displacement in the same direction as the
applied force; if the force is constant and the displacement is at an angle θ to the force the W=Fdcosθ
-power: rate at which work is done; expressed in watts (1 watt= 1W= 1J/s); horsepower is 746 watts
-potential energy: U, energy associated with arrangement of objects in a system (bow and arrow, fault
lines in relation to earth quakes); gravitational PE energy available from gravitational force btwn
objects; elastic PE energy available in an extended or compressed spring; mechanical PE is sum of an
objects kinetic and potential energy
-GPE: changes in gravitational pe dependent on height, h ( ∆U in Joules); if object moves up an incline,
change in pe is only due to how high not how far you move -EPE: depends on how far you stretch a spring and how stiff said spring is, which is determined by the
spring constant k, where k is in N/m² (the larger k is the stiffer the spring); in Joules for ∆U
-MPE: sum of KE+PE; if system is isolated and only conservative forces (gravity, ideal springs), are
present then ME is preserved!
-work done by friction: push a block on surface with significant friction hard enough, and thermal energy
is released (heat, due to work); source of thermal energy is friction; in general, block could be moving up
ramp or attached to string; f k is increase in thermal energy due to friction
-linear momentum: defined for a particle as p=mv; momentum is related to force bc acceleration is the
change of velocity with time so force=rate of change of momentum with time; this is how newton
originally expressed his second law; in an isolated system we know the total energy is constant, but can
manifest itself in many forms (KE, PE, internal energy, thermal energy); mechanical energy may not be
conserved in an isolated system, but momentum always is conserved; momentum is therefore useful in
situations where N2 is not, such as collisions (hitting a bump etc)
Mountain Bike Suspension
-suspension: plays an important role, even on smooth surfaces, for vehicle handling and performance;
also improves ride comfort; no matter intended use, basic goal of suspension is to dissipate external
forces acting on the vehicles occupants; bicycles suspension is to eliminate or reduce the bumps or
vibrations enco

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